Rotary mowers currently predominate over other types of mowers, particularly in the home-owner field for mowing lawns. Such a mower conventionally comprises a housing in which a rotor is mounted for rotation about a generally vertical axis. The rotor has outer cutting edges traveling in a generally horizontal plane. Sometimes the cutting plane is slightly tilted downwardly toward the front of the mower, with a corresponding tilt of the vertical axis. Such tilts are included herein within the horizontal and vertical terminology. Also the terms are with respect to ground, since both level and sloping surfaces may be mowed.
The housing conventionally has a deck with a downwardly extending skirt. A gasoline or electric motor is mounted on the deck, with a vertical shaft to which the rotor is attached. In tractor units, power takeoff may be provided from the tractor motor.
In rotary mowers cutting is accomplished by the high speed impact of the cutting blades on the grass. In the past, tip velocities exceeding 20,000 feet (6100 meters) per minute have often been employed. Such mowers have been recognized to be highly dangerous to the feet (or hands) of a user, and to nearby persons since stones, etc. may be thrown with great velocity. Nevertheless, the relative simplicity and lower cost as compared to reel and other types of mowers, the greater ease of sharpening, and the ability to cut tall as well as short grass, has resulted in widespread use of the rotary mower.
In recent years efforts have been made to reduce the danger of rotary mowers. Thus, lower tip velocities have been employed and the skirt extended below the cutting plane of the blades. Such changes have commonly resulted in poorer mowing performance. Various expedients have been employed to improve the mowing. One expedient is the use of lift blades to create an up draft to stand up the grass so that it can be cut more effectively, and to provide an air flow to remove the clippings and avoid clogging the mower. In the lift blades, trailing portions of the cutting blade tips are tilted upwardly.
Unfortunately the lift created by the upward tilt is accompanied by a "blow down" due to the tip vortex generated as a result of the lift. This is air rushing downward off the end of the blade tip to neutralize the low pressure created under the end of the blade by the lifting action. In general, the greater the lift, the greater the blow down. Such blow down tends to blow grass away from the tip, and thereby reduces the cutting effectiveness at the tip.
The blow down can be reduced by closely spacing the skirt of the mower to the tip circle of the rotating blades or inserting a closely spaced wall in the housing, and extending the skirt and/or wall below the blade tip. However, the skirt or wall then impairs the ability of the grass to reach the blade tip, particularly tall grass. One result is to reduce the effective width of cut of the mower. For example, a mower with a 21 inch (53.5 cm) diameter blade may cut a flat swath only 15 to 18 inches wide (38 to 46 cm). Another result is the tendency in moderately high grass to leave uncut or partially uncut grass under the front half of the blade circle, relying on completing the cutting as the rear half of the blade circle passes over the grass. Thus covering the bottom of the rear half of the blade circle may markedly impair the finished appearance of the lawn. The build up of pressure in the space between the rotor and the deck above it may further increase the blow down problem. Also, with high lift blades, circumferential air flow may cause the grass to bend down in the direction of blade travel.
It has also been suggested to pivotally mount metal cutting blades on a blade carrier so that they can swing back upon striking an object. So far as we are aware, such blades are still capable of exerting very high forces on an object, and severe damage to the cutting edges may result.
Flexible cutting elements such as filaments of nylon, etc. have been employed to reduce hazards, mounted on a rotor and driven at high speeds so as to provide cutting edges traveling in an essentially horizontal plane during mowing. In general such mowers are believed to be less than satisfactory under various mowing conditions encountered in practice, and have disadvantages of their own. Among these are difficulties in handling the grass clippings.
Other expedients have been suggested to reduce the danger of rotary mowers while preserving adequate mowing performance. Frequently more complicated and expensive structures have resulted and/or the resultant performance leaves much to be desired.
U.S. Pat. No. 3,673,773 to Hugo S. Ferguson, one of the present inventors, discloses a cross flow mower employing a cylindrical reel rotating about a horizontal axis. U.S. Pat. No. 3,818,684, also to Ferguson, gives further details of the air flow in such mowers, including the vortex generated inside the reel and the inlet flow of air. In such mowers the blower blades have their leading edges sharp to form the cutting edges for mowingthe grass. The internal vortex flow is in generally vertical planes spaced along the reel axis, and the flow of air in the inlet region is in generally vertical planes perpendicular to the reel axis with upward components perpendicular to the reel axis.
Although the aforesaid functioning of the horizontal reel cross flow mower would appear to render cross flow operation inapplicable to rotary type mowers, it has now been found possible to incorporate cross flow blower action in a rotary mower in such a manner as to greatly improve the safety of the mower while preserving excellent mowing performance and handling of clippings without plugging.